The present invention relates to a multimode optical fiber. Such fibers are used especially for short-distance optical transmission systems which require a wide bandwidth.
An optical fiber is conventionally composed of an optical core, which has the function of transmitting the light pulse of an optical signal, and an optical cladding, which has the function of confining the optical signal within the core. For that purpose the refractive indices of the core n1 and of the cladding n2 are such that n1>n2.
Generally, in a fiber there are defined an axial direction (Y), which corresponds to the propagation direction of the optical signal in the fiber, a radial direction (r), which extends from the center of the core to the cladding of the fiber, and a transverse plane (X, Y), which corresponds to a section through the fiber perpendicular to its axial direction.
Multimode fibers are frequently used for short-distance applications and for local networks. A multimode core generally has a diameter of about 50 μm, as opposed to about 6 μm for a single-mode core. As a result, for a given wavelength, a plurality of optical modes propagate simultaneously along the fiber, carrying the same information.
The bandwidth is directly linked to the group delay of the optical modes propagating in the multimode core of the fiber. In order to ensure a wide bandwidth it is necessary for the group delays of all the modes to be the same, that is to say for the intermodal dispersion to be zero or at the very least minimised, for a given wavelength.
However, in a conventional stepped-index fiber, the various modes propagate at different velocities along the fiber, which causes spreading of the light pulse, which can approach the spacing between the pulses and give rise to an unacceptable error rate.
In order to decrease the intermodal dispersion in a multimode fiber, it has been proposed to produce graded-index fibers, an illustration of which is provided in FIG. 1. A graded-index fiber comprises a multimode core 10 having a radial index profile and a confinement cladding 30. Such a fiber is defined and its characteristics described in the publications “Multimode theory of graded-core fibres” by D. Gloge et al., Bell system Technical Journal 1973, pp 1563-1578, and “Comprehensive theory of dispersion in graded-index optical fibers” by G. Yabre, Journal of Lightwave Technology, February 2000, Vol. 18, No. 2, pp 166-177.
A graded-index profile can be defined by a relationship between the value n of the index at a point as a function of the distance r of that point from the center of the fiber:n2(r)=nmax2{1−2Δ(r/rmax)α}
wherein α≧1 (α→∞ corresponding to a step in the index);                nmax is the maximum index of the multimode core;        rmax is the radius of the multimode core;        Δ=(nmax2−nmin2)/2nmax2; wherein nmin, the minimum index of the multimode core, generally corresponds to the index of the cladding.        
To summarize, a graded-index fiber has an index profile in the multimode core which has symmetry of revolution and is such that along any radial direction the value of the index decreases continuously from the center of the fiber to its periphery.
When a multimode light signal propagates in a graded-index core of such a kind, the different modes “see” a different propagation medium, which differently affects their propagation velocity. By adjusting the value of the parameter α, it is accordingly possible to obtain a group velocity which is virtually equal for all the modes and, therefore, reduced intermodal dispersion.
Such a solution has two major disadvantages, however. Firstly, the index gradient is obtained by controlled doping of the multimode core, for example doping of the silica or plastics-material core with germanium, which calls for a complex and costly manufacturing process. Secondly, changes in the doping with ageing of the fiber can cause appreciable deterioration in intermodal dispersion, especially in the case of plastics-material optical fibers.
Also known, from the document EP 1 199 581, is an optical fiber having a microstructured multimode core, an illustration of which in a cross-sectional view is provided in FIG. 2. This optical fiber has an equivalent index gradient introduced into the multimode core by elements axially oriented along the length of the fiber. These elements, for example air holes, are provided circumferentially around the center of the multimode core. A microstructured fiber of such a kind is, however, complicated to manufacture.
In addition, multimode fibers also have applications as dual-core fibers for optical amplifiers or lasers. A single-mode central core in that case allows an optical signal to be transmitted and a multimode core allows a pump signal to be injected. A doped region, for example doped with rare earth elements, is arranged within the single-mode core or as a ring in a region surrounding said core, as is known from the document U.S. Pat. No. 6,288,835. The optical signal of the single-mode core is amplified by interaction with the pump signal passing through the doped region. The efficiency of amplification depends directly on the overlap between the pump signal and the single-mode signal. Therefore, the multimode pump signal has to be made to pass through the single-mode core as often as possible along the doped fiber.